The tumor suppressor gene p53 encodes a protein having a variety of functions such as the induction of apoptosis, cell cycle arrest, the repairing of DNA damages, and the like. It has also been shown that p53 is a transcription factor which regulates the expression of various proteins. Currently this protein is believed to play a central role in the control of cell growth. It has also been reported that p53 mutation is observed in about half of tumor cells which is mainly responsible for abnormal growth and resistance against anti-cancer agents. Although no proteins were known that have a p53-like structure and function until very recently, but two novel p53-like molecules were recently reported. One is p73 (Cell, 90:809–819, 1997), and the other is p51 (Nature Medicine, 4:839–843, 1998). Concerning p51, a protein encoded by the same gene is reported as p63 (Molecular Cell, 2:305–316, 1998). Structurally p51 is very similar to p53, and like p53 it is capable of activating the transcription of a cell cycle progression suppressor protein p21, suggesting that it is homologous to p53 functionally as well. Furthermore, it is reported that p51, as is p53, is capable of inducing cell death, suppressing cell growth, and the like. It has been shown, however, that p51 is different from p53 in that no mutation in p51 has been observed in tumor cells. It is being elucidated that p51 is expressed in very limited organs such as muscle cells.